在 JButton 不更新的情况下更改 JFrame 背景颜色

Question

I'm trying to make a very simple GUI that updates the background color upon clicking a button. I can't for the life of me figure out what is wrong with my code. When it is run nothing updates upon clicking the buttons, any help would be appreciated!

import java.awt.event.*;
import javax.swing.*;
import java.awt.*;

public class ChallengeGUI extends JFrame implements ActionListener {

    private static final long serialVersionUID = 1L;
    private JButton blue;
    private JButton red;
    private JButton green;


    public ChallengeGUI() {

        JButton blue = new JButton("BLUE");
        blue.addActionListener(this);
        add(blue);

        JButton red = new JButton("RED");
        red.addActionListener(this);
        add(red);

        JButton green = new JButton("GREEN");
        green.addActionListener(this);
        add(green);

        setLayout(new FlowLayout());
        setSize(600,600);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == blue) {
            getContentPane().setBackground(Color.BLUE);
        } else if (e.getSource() == red) {
            getContentPane().setBackground(Color.RED);
        } else if (e.getSource() == green) {
            getContentPane().setBackground(Color.GREEN);
        }

    }

    public static void main(String[] args) {
        new ChallengeGUI();

    }

}

Answer 1

You have one mistake.

• You've shadowed your buttons so e.getSource()==red is always false.

Instead of

JButton red = new JButton("RED");

just write

red = new JButton("RED");

When you wrote: JButton red =... it created a local variable named red separate from the field declared in your class, also named red . That means the field did not get initialized. In your listener, none of the cases were true because red , blue , and green were null and e.getSource() was returning the local JButton you created.

Answer 2

You can write your actionPerformed() method like this:

public void actionPerformed(ActionEvent e) {
    String actionCommand = e.getActionCommand();
    Color color;
    switch (actionCommand) {
        case "BLUE":
            color = Color.BLUE;
            break;
        case "RED":
            color = Color.RED;
            break;
        case "GREEN":
            color = Color.GREEN;
            break;
        default:
            color = null;
            JOptionPane.showMessageDialog(null, "Unhandled: " + actionCommand);
    }
    if (color != null) {
        getContentPane().setBackground(color);
    }
}